1. Fields of the Invention
The present invention relates to a manufacturing method of the organic solar cell, especially by means of an application about the photoresist spin-coating and photolithograph technologies to balance the thickness and flatness of the organic active layer, and it effectively enhances the photoelectric conversion efficiency of the organic solar cell.
2. Descriptions of Related Art
The sun is the source of life, and human being cannot live without the sun. Although there are no immediately exhausted crises for the fossil fuels, e.g. oil or coal, on which the life around the world rely, the carbon dioxide emission from the excessive use of the fossil fuel has already caused the serious greenhouse effect to become the culprit in the earth's warming temperatures. Furthermore, as the price of crude oil continued to rise in recent years, looking for alternative energy sources has become imperative. Alternative energy sources, such as wind, hydro, geothermal, biodiesel, solar cells and so on, to be called as green energy, have attracted considerable attention over recent years, among which the solar cell is the most promising due to its high theoretical efficiency and mature technology.
The solar cell can transform the solar energy into electrical energy based on the photoelectric effect of materials. The photoelectric effect is the phenomenon that light shines into the material to increase conductive carriers. In terms of the semiconductor materials, as the energy of the light is larger than the energy gap of the semiconductors, the free elector-hole pairs are generated in the interior. However, these elector-hole pairs can be recombined soon or captured by the carriers in the semiconductors to become vanished. If an internal electric field is applied at this time, the carriers will be quickly led out before vanished. The internal electric field is generated in the joint interface between p-type and n-type semiconductors, and a so-called solar cell uses the internal electric field to extract effectively the current to induce the electricity.
According to the efficiency and production costs of the solar cell, the solar cell can be divided into three generations in development thereof, the first generation solar cell mainly made of silicon wafers with an efficiency over 20% shows disadvantages of a higher production cost and difficultly refining the silicon wafers, which results in a second generation solar cell. The second generation solar cell is mainly made for reducing the silicon content or made of cheaper materials, e.g. Cadmium Antimonide (CdTe) and Copper Indium Selenide (CuInSe2) or the like, with an efficiency identical to that of the first generation. The third generation solar cell is developed for two specific purposes, one for an high conversion efficiency and the other for reduction of production costs by creation of an appropriate efficiency ranging from 15% to 20%. In terms of the conversion efficiency enhancement, although the solar cell having a conversion efficiency up to 30% by use of the multi-layers of different semiconductor materials to absorb respectively sunlight of different wavelengths has been already developed, its high production cost is still an issued to be unresolved. On the other hand, using the organic materials to produce the third generation solar cell shows the advantages of lower production costs and simpler processes. Although the conversion efficiency of the solar cell is low, it has a very good potential in application and development thereof.
The biggest advantage of the polymer organic solar cell is to use the solution process, such as spin coating and inkjet coating methods. In an existing technology, the bulk heterojunction blend process is most commonly used in production thereof. Particularly, a solution blended with conjugated polymer (P3HT) as the donor and phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor is widely used to form an organic active layer over a substrate by a spin coater, a prepared solution for the organic active layer is dropped to form an uniform film on the substrate by a centrifugal force. There are only two deposition parameters in the spin coater, one for the spin-coating speed and the other for the spin-coating time. When the spin-coating speed gets higher, the film will get thinner and more uniform. However, obtaining a thinnest or thickest film will become an issue to be resolved. For example, the thinner the film is, the higher the spin-coating speed is, but subject to a speed limit of the spin coater; on the contrary, the thicker the film is, the lower the spin-coating speed is, but subject to unfavorable conditions of the film not spinning out at a lowest spin-coating, the liquid having a viscous force to make the four sides of the substrate become thicker, and the liquid flowing back to the inside of the substrate to make an uneven film in thickness when the spin coater stops. In researches, it is found that when the organic active layer gets thicker, there is a higher current density in measurement of current-voltage characteristics, which can further infer that there are many electron-hole pairs generated from the organic active layer. However, due to the unfavorable conditions occurred at a lower and lower spin-coating speed as aforesaid, the surface of the active layer will have a higher roughness. Also, a higher current density is easy to cause the defects of the organic active layer, so that fill factors (FF) will become poorer in the organic active layer and the overall efficiency of the solar cell will be relatively reduced. When the spin-coating speed for coating the organic active layer increases, the fill factors will increase to make the overall efficiency of the solar cell relatively stable, but the thickness of the organic active layer will be reduced to further result in the current density reduction. Thus there is a need for the organic solar cell vendors and educators to find a solution for an organic solar cell having a high performance under the conditions of unchanging the original materials of the organic active layer, obtaining a higher current density and promoting the fill factor efficiency.